The present invention relates to tubing for conveying fluids therethrough. More particularly, the present invention related to a polymeric tubing which is resistant to permeation by organic compounds. The present invention also relates to tubing adapted for use in a motor vehicle. More particularly, the present invention relates to a multi-layer tube which can be employed for transporting hydrocarbon fluids such as a fuel line or vapor recovery line of a motor vehicle.
There exists a need for durable polymeric tubing in a number of fluid handling situations. Depending on the nature of the fluid conveyed through the tube and/or the external environment in which the tube is placed, there is also a need for the polymeric tube to exhibit low permeation to various organic compounds, for example, hydrocarbons.
One area where polymeric tubing can be advantageously employed is in motor vehicles. Single layer fuel lines and vapor return lines of synthetic materials such as polyamides have been proposed and employed in the past. Fuel lines employing such materials generally have lengths of at least several meters. It is important that the line, once installed, not materially change during the length of operation, either by shrinkage, elongation or as a result of the stresses to which the line may be subject during use.
It is also becoming increasingly important that the lines employed be essentially impervious to hydrocarbon emissions due to permeation through the tubing. It is anticipated that future Federal and state regulations will fix the limit for permissible hydrocarbon emissions due to permeation through such lines. For instance, various environmental standards mandate maximum levels for total passive hydrocarbon emission for a vehicle. Levels at or below 2 g/m2 per 24 hour period as calculated by evaporative emission testing methods such as those outlined in Title 13 of the California Code of Regulations, section 1976, proposed amendment of Sep. 26, 1991 can be required. To achieve the desired total vehicle emission levels, a hydrocarbon permeation level for the fuel and vapor lines equal to or below 0.5 g/m2 per 24 hour period would be required. Finally, it is also important that the fuel line employed be essentially impervious to interaction with corrosive materials present in the fuel such as oxidative agents and surfactants as well as additives such as ethanol and methanol.
Various types of tubing have been proposed to address these concerns. In general, the most successful of these have been co-extruded multi-layer tubing. The successful tubing constructions generally have a relatively thick outer layer composed of a material resistant to the exterior environment. The innermost layer is thinner and is composed of a material which is chosen for its ability to block diffusion of organic materials to the outer layer. The materials blocked typically are organic compounds such as aliphatic hydrocarbons, alcohols and other materials present in fuel blends. The materials of choice for the inner layer are polyamides such as Nylon 6, Nylon 6.6, Nylon 11 and Nylon 12.
Alcohol and aromatics in the fluid conveyed through the tube diffuse through the tubing wall at rates different from the aliphatic components. The resulting change in the composition of the liquid in the tubing can change the solubility thresholds of the material in the tube so as, for example, to be able to dissolve monomers and oligomers of tubing materials, such as Nylon 11 and Nylon 12, into the liquid. The presence of copper ions, which can be picked up from the fuel pump, accelerates crystallization of the dissolved monomers and oligomers into a crystalized precipitate. The crystallized precipitate can block filters and fuel injectors and collect to limit travel of the fuel-pump or carburetor float as well as build up on critical control surfaces of the fuel pump.
In U.S. Pat. No. 5,076,329 to Brunnhofer, a five-layer fuel line is proposed which is composed of a thick outer layer formed of Nylon 11 or Nylon 12, a thick intermediate layer of Nylon 6, and a thin intermediate bonding layer between and bonded to the intermediate and outer layers formed of a polyethylene or a polypropylene. On the interior of the tube is an inner layer of Nylon 6 with a thin intermediate solvent-blocking layer formed of an ethylene-vinyl alcohol copolymer transposed between. The use of Nylon 6 in the inner fluid contacting surface is designed to eliminate at least a portion of the monomer and oligomer dissolution which occurs with Nylon 11 or Nylon 12.
In U.S. Pat. No. 5,038,833 to Brunnhofer, a three-layer fuel line is proposed in which a tube is formed having a co-extruded outer wall of Nylon 11 or Nylon 12, an intermediate alcohol barrier wall formed from an ethylene-vinyl alcohol copolymer, and an inner water-blocking wall formed from a polyamide such as Nylon 11 or Nylon 12. In DE 40 06 870, a fuel line is proposed in which an intermediate solvent barrier layer is formed of unmodified Nylon 6.6 used either separately or in combination with blends of polyamide elastomers. The internal layer is also composed of polyamides; preferably modified or unmodified Nylon 6. The outer layer is composed of either Nylon 6 or Nylon 12.
Another tubing designed to be resistant to alcoholic media is disclosed in UK Application Number 2 204 376 A in which a tube is produced which has a thick outer layer composed of polyamides such as Nylon 6 or 6.6 and/or Nylon 11 or 12 which are co-extruded with an alcohol-resistant polyolefin, a co-polymer of propylene and maleic acid.
Heretofore, it has been extremely difficult to obtain satisfactory lamination characteristics between dissimilar polymer layers. Thus all of the multi-layer tubing constructions proposed previously have employed polyamide-based materials in most or all of the multiple layers. Materials employed typically have been polyolefin-based polymers which have a high affinity to polyamide. While many more effective solvent-resistant chemicals exist, their use in this area is limited due to limited elongation properties, strength and compatibility with Nylon 11 and 12.
Thus, it would be desirable to provide an improvement in a tubing material which could be employed in motor vehicles which would be durable and prevent or reduce permeation of organic materials therethrough. It would also be desirable to provide a tubing material which would be essentially nonreactive with components of the liquid being conveyed therein.
The present invention is a multi-layer tubing for conveying fluids therethrough conveying fluids containing hydrocarbons. The invention can be used in a motor vehicle system to transport fluids such as would be found in a fuel line, a vapor return line or a vapor recovery tube. The multi-layer tube in the present invention comprises an inner layer having an inner surface capable of prolonged exposure to a fluid containing hydrocarbons, the inner layer consisting essentially of an extrudable, melt-processible thermoplastic. The invention further comprises an outer thermoplastic layer and at least one intermediate layer. The intermediate layer is composed of a solidified thermoplastic solution or solidified suspension in which at least one thermoplastic compound is present as a continuously and discretely distributed element in a matrix consisting of a second discrete thermoplastic material.
Other applications of the present invention will become apparent to those skilled in the art when the following description of the best mode contemplated for practicing the invention is read in conjunction with the accompanying drawings.